In the practice of orthodontic medicine, the position of teeth within the mouth is altered by anchoring tension wires to two or more teeth and applying sustained, steady pressure to the teeth to be moved. Tension wires are attached to individual teeth by surrounding the tooth with a band or by cementing a bracket to the tooth using an adhesive. If bands are used, they must be stocked by the orthodontist in many sizes. Also, bands tend to promote dental caries adjacent to the band.
Orthodontic brackets have become popular because they are simpler to apply to teeth than bands, see Retief and Denys, "Finishing of Enamel Surfaces After Debonding of Orthodontic Attachments", The Angle Orthodontist, Vol. 49, No. 1, pp. 1-10 (January 1979). After treatment, the bracket is removed from the tooth using orthodontic pliers or ligature cutters, and any residual adhesive is removed by grinding or scraping.
Early work with bracket adhesives dealt with means for improving the bond between the tooth and bracket in order to prevent inadvertent early failure of the adhesive bond. However, with improvements in tooth preparation, formulation of the adhesive, and construction of brackets, early bond failure is no longer a major problem. Instead, attention has now focused on techniques for minimizing tooth enamel damage when the bracket and any residual adhesive is removed. Modifications in dental technique designed to reduce such damage are described in Retief and Denys, id; Burapavong and Apfel, "Enamel Surface Characteristics on Removal of Bonded Orthodontic Brackets", Am. J. Orthod., Vol. 74, No. 2, pp. 176-187 (August 1978); Bennett, Schoen and Going, "Alterations of the Enamel Surface Following Direct-Bonded Bracket Therapy", Journal of Pedodontics, Vol. 4, No. 2, pp. 99-113 (Winter 1979); and Zachrisson and Arthun, "Enamel Surface Appearance After Various Debonding Techniques", Am. J. Orthod., Vol. 75, No. 2, pp. 121-137 (February 1979).
Studies have indicated that tooth preparation (by acid etching), removal of the bracket and adhesive, and polishing the tooth surface typically removes from about 43 to 56 microns of tooth enamel, see Fitzpatrick and Way, American Journal of Orthodontics, Vol. 72, No. 6, pp. 671-680 (December 1977) and Brown and Way, American Journal of Orthodontics, Vol. 74, No. 6, pp. 663-671 (December 1978). Although a loss of 55 microns of enamel represents only about 3 percent of the total enamel thickness of 1500 to 2000 microns, the highest concentration of fluoride is contained in the outer 20 microns of enamel. Also, any gouges or scratches which occur during removal may lead to later plaque and caries formation at the site of the scratch. Current recommendations for minimization of enamel damage are to use a tungsten carbide burr at low speed in Zachrisson and Arthun, id, a green rubber wheel (contraindicated somewhat due to generation of heat) in Gwinnett and Gorelick, Am. J. Orthod., Vol. 71, No. 6, pp. 651-665 (June 1977), or a low viscosity unfilled resin with a wire mesh base bracket (contraindicated somewhat due to undesirable slippage and shifting of the bracket on the tooth during curing) in Dogon and Moin, "An Evaluation of Shear Strength Measurements of Unfilled and Filled Resin Combinations", Am. J. Orthod., Vol. 74, No. 5, pp. 531-536 (November 1978). These references indicate that a need exists for an orthodontic bracket adhesive system which can be removed without excessive damage to tooth enamel.
Existing orthodontic bracket adhesives are generally made from binder, filler, and compounding and coloring adjuvants. Typically, the binders are the same as the binders used in tooth filling materials (i.e., dental restorative resins), such as the diglycidyl methacrylate adduct of epoxy resin (BIS-GMA) described in U.S. Pat. No. 3,066,112.
The fillers used in orthodontic bracket adhesives impart a sufficient viscosity to the adhesive to make it readily workable in the mouth. Without such a filler, the adhesive or the bracket is prone to slippage prior to cure. The fillers used in orthodontic bracket adhesives are generally also used in some dental restorative resins and include quartz, silica gel, aluminum silicate, silica, glass beads, aluminum oxide, titanium dioxide, zirconia, spodumene, lithium aluminum silicate, barium aluminum silicate, and silicate or phosphate glasses, see U.S. Pat. Nos. 3,625,916, 3,629,187, 3,709,866, 3,895,445, 4,010,545, and 4,063,360, and U.K. Published Patent Application No. 2,006,792 A. Some references have suggested using lower amounts of filler in an orthodontic bracket adhesive than the amounts typically used in those restorative resins containing such fillers in order to ease removal of excess adhesive, see Gwinnett and Gorelick, id, Retief and Denys, id, and U.S. Pat. No. 3,629,187. However, removal of residual adhesive containing these fillers still can lead to damage to tooth enamel, because hardened tools must be used to grind away the excess adhesive.
The hardness of tooth enamel, measured upon the Mohs scale of hardness, is approximately 4.5 to 5, depending upon the age and location of the tooth. The fillers used in currently marketed orthodontic bracket adhesives have a Mohs hardness well in excess of this value. Consequently, yet harder abrasives must be used to remove such adhesives. Currently used abrasives include silicon carbide, diamond, stainless steel, aluminum oxide, and pumice. Such abrasives are described, for example, in Greener, Harcourt and Lautenschlager, "Materials Science in Dentistry", Williams & Wilkins, Baltimore (1972) at, e.g. pp. 379-383, Craig, O'Brien and Powers, "Dental Materials, Properties and Manipulation" Mosby, St. Louis (1979) at e.g. Chapter 6, and Anderson, "Applied Dental Materials", Blackwell, Oxford (1972) at, e.g. Chapter 29. All of these abrasives readily abrade tooth enamel.
In U.S. Pat. No. 4,141,144 (Lustgarten) there are described compositions for use as "paint-on" tooth veneers and direct filling materials, said to be useful for matching the hue and lustre of treated teeth with the hue and lustre of untreated teeth. Such compositions contain about 1 to 85 percent by weight muscovite mica flakes having an average size of less than about 50 microns. In such compositions some or all of the muscovite mica flakes are preferably coated with a continuous layer of TiO.sub.2 or ZrO.sub.2 (which coating materials have Mohs hardnesses of 51/2 to 61/2 and 7 to 71/2 respectively), or BiOCl (which coating material has an unreported Mohs hardness, due perhaps to the commercial unavailability of crystalline BiOCl in a crystal size sufficient to allow Mohs hardness testing to be carried out). Also, such compositions preferably contain inorganic filler such as silica, glass beads, aluminum oxide, fused silica, fused or crystalline quartz, or the like (which inorganic fillers have generally high Mohs hardnesses, e.g., 7 or more). Such compositions therefore preferably contain inorganic materials whose surfaces are harder than tooth enamel. In the only example in Lustgarten in which uncoated mica is used and coated mica and inorganic filler are excluded (viz., Example 14) about 22 percent by volume (about 39 to 41 percent by weight) uncoated muscovite mica was combined with methyl methacrylate to form a thin, yellowish 1.5 mm thick cured slab which was used in various optical comparison tests.
Dental restorative resins containing a filler having a Mohs hardness of 3 to 5 have been described as offering good wear resistance and polishability in U.S. Pat. No. 4,020,557, and dental restorative resins containing a rod-like filler having a Mohs hardness of 3.5 to 6 (such as calcium silicate) have been described as offering a resin with good strength and workability in Australian Patent Specification No. 50674/73. These references do not suggest the use of such fillers in an orthodontic bracket adhesive and abrasive system.
The above cited U.K. Published Patent Application No. 2,006,792 A states that an organic filler such as polymethyl methacrylate can be used in an orthodontic bracket adhesive. Such an organically filled adhesive has low hardness but insufficient tensile strength.